Various agents, such as radiation, ultraviolet light, synthetic chemicals, natural substances, and aberrations in genetic replication and repair can produce mutations in DNA. The results of a representative study indicate that as many as 60% of the cancers that develop in women and as many as 40% of those that develop in men result from avoidable exposure to mutagens from dietary intake. Vuoto et al., Environ. Mutaqen, 7:577-598 (1985). Exposure to environmental mutagens such as nitroaromatic compounds found in automobile exhaust, chlorination by-products used in drinking water, and acrylamide and formaldehyde used extensively in industrial laboratories is also of major concern. Quantitative measurement of the effect of suspected mutagens is essential to control exposure to harmful agents. Additionally, whenever a new chemical, drug, or food additive, for example, is to be taken from the laboratory to the marketplace, it must be tested for its toxicity and cancer-causing potential. As a result, significant effort has gone into the development of assays that detect the mutagenic potential of various compounds.
Existing tests that assess the mutagenic potential of substances focus either on alterations of DNA in cultured cells or bacteria or alterations in the health of test animals. However, few tests that monitor alterations in DNA actually expose live animals to the agent to be tested. This is because it is very difficult to rapidly monitor alterations in the genetic code simultaneously in many different organs. Tests to detect these mutations must be very sensitive. They must be able to detect a single mutation amongst millions of normal genetic units. The difficulty of this task currently makes this approach for live animal studies prohibitively expensive as well as time intensive. Therefore, most current live animal genotoxicity tests use disease formation or large scale chromosomal alterations as an assay for gene alteration.
The problem of readily detecting small scale DNA alterations that are caused by potential mutagenic agents has generally been approached by performing studies on procaryotic or eukaryotic cells in culture (in vitro tests). The well-known Ames' test uses a special strain of bacteria to detect these mutations. Ames, et al., An Improved Bacterial Test System for the Detection and Classification of Mutagens and Carcinogens, Proc. Nat. Acad. Sci., 70:782-86 (1973). This test and many analogues that use other types of bacterial or animal cells permit the rapid screening of very large numbers of cells for the appearance of an altered phenotype. The appearance of this altered phenotypic trait reflects the occurrence of a mutation within the test gene. These tests are, however, insensitive to or nonspecific for many mutagens that result from metabolic activation of the agent being screened. Although attempts have been made to increase their sensitivity and specificity by activation of such metabolites with liver and other extracts it is noted that, for instance, the metabolites produced by these extracts are often not present at the same concentrations as in the live tissues of an animal. Metabolites that are only produced in other organs are not detected at all.
Eukaryotic cell lines have also been used to detect mutations. E.g., Glazer et al., Detection and Analysis of UV-induced Mutations in Mammalian Cell DNA using Lambda Phage Shuttle Vector, Proc. Natl. Acad. Sci. USA, 83:1041-1044 (1986). In this test a target test gene, the amber suppressor tyrosine tRNA gene of E. coli in a bacteriophage shuttle vector, was integrated into a genomic host mammalian cell line by DNA transfection of cultured cells in vitro. After exposing the host cell line to putative mutagenic agents, test genes were reisolated, propagated in bacteria, and analyzed for mutations. Because the host is only a mammalian cell line and not a live animal, the test is incapable of accurately monitoring mutagenic metabolites of the agent being tested that are only produced at the appropriate concentrations by differentiated cells or the tissue of live animals.
A two year study by the NIH concluded that data obtained from four different prokaryotic and eukaryotic in vitro assays had only a 60% concordance with whole animal carcinogenicity studies. Tennant et al., Science, 236:933-941 (1987). The study suggests that the high rate of error may result from potential variation in genetic susceptibility between in vitro systems and whole animals. For example, metabolites, frequently involved in activation of promutagens, are not present in in vitro systems, allowing mutagenic potential to go undetected. In addition, differences in DNA repair mechanisms between prokaryotes and eukaryotes may account for some discrepancies in results.
Test genes and large scale screening assays used for in vitro assays are not available for live animal studies. Short of relying on long term animal studies that detect phenotypic changes that require a long time to be identifiable, such as tumors, organ failure, coat color, etc., current tests do not provide a means for monitoring organ-specific mutations of DNA. Hence, there exists a need for a system that places a test DNA sequence within an animal and is subsequently assayed on a large scale for mutations. There also exists a need for methods that detect mutations caused by chemical metabolites of the agent being tested. To be most effective the system needs to be capable of monitoring genetic changes in as many tissues of an animal and as easily, rapidly, and inexpensively as possible.
The present invention, providing novel transgenic non-human mammals and methods utilizing such mammals for mutagenesis testing, satisfies these needs. More specifically, the present invention provides a sensitive screen for the mutagenicity of suspected agents and permits the monitoring of the mutagenic effects of such agents and the mutagenic effects of the metabolites of such agents. Additionally, the invention can permit the identification of the nature of the mutation, e.g., DNA transition, transversion, deletion, or a point or frameshift mutation. Further, the methods of the invention offer the significant advantage of being rapid to perform, thus permitting the identification of potential mutagens appreciably before other tests can be completed, and is inexpensive relative to other whole animal tests. And, the present invention substantially reduces the number of animals which must be used for mutagenesis testing.